xe2x80x9cThis application is a national phase of PCT/FR00/02176 which was filed on Jul. 28, 2000, and was not published in English.xe2x80x9d
The present invention concerns a structure for a lithographic reflection mask, in particular for extreme ultra-violet lithography (EUV). Such a mask may be used particularly in the manufacture of integrated circuits.
The invention also concerns a process for manufacturing a structure for a lithographic reflection mask.
Lithography or photo-lithography is the process which allows the image of a mask to be transferred into a layer of photo-sensitive resin deposited on a sample. This exposure stage of a resin through a mask, by means of a light source and an optical system, is achieved on so-called photo-masking equipment. After exposure, the resin is revealed thus making it possible to obtain on the sample a resin mask which will be used in the stages required in the manufacture of a micro-electronic device (engraving mask, depositing mask or ion implantation mask).
Progress in optical lithography has been the key factor in the growth of the integrated circuit (IC) industry over recent years, the main vector of improvement being the use of a shorter and shorter wavelength for exposing the resins. Thus, lithography has gone from 365 nm to 248 nm and today to 193 nm, the reduction in wavelength allowing the critical dimensions of ICs to be reduced to the point where they achieve dimensions of the order of a tenth of a xcexcm. This technical progress and the availability, in time, of production facilities for photo-lithography, masks and resins has made it possible, for 30 years, for the micro-electronics industry to continue its growth in accordance with Moore""s Law.
However, the pursuit of the miniaturisation of integrated circuits with sub-0.1 xcexcm patterns requires a new technological approach at odds with optical lithography. Indeed, the refractive materials currently used for optical lithography become too absorbent for wavelengths below 190 nm. Different solutions known collectively as New Generation Lithography or NGL have been proposed. The stakes are high for the semi-conductor industry.
Among NGL, Extreme Ultra-Violet Lithography (EUV) using wavelengths of between 10 and 14 nm (soft X-ray radiation) is one of the most promising avenues. At these short wavelengths, resolution of below 0.1 xcexcm may be obtained, while retaining a low numerical aperture and a depth of field above 1 xcexcm. This technology may satisfy several sub-0.1 xcexcm generations of technology until the 256 Gbit generation planned for production in 2012 by the Semiconductor Industry Association (SIA). EUV lithography is the solution prioritised by the international association of semi-conductor industries (International Sematech). Significant work is in progress on this in the United States, in Japan and in Europe.
The technological challenges set by the implementation of lithography in the extreme ultra-violet concern five aspects in particular: the EUV radiation source, the projection optics of the photo-masker, the masks, the resin processes and the metrology associated with the development of the optics and the masks.
Unlike current optical lithography masks (transmission masks made by depositing chrome on a quartz base) a mask for EUV lithography is a mask in reflection and no longer in transmission.
FIG. 1 is a transverse cross-section view of an EUV mask according to the prior art. This mask is composed of a substrate 1 on which is deposited a reflective coating 2 which is in its turn coated with an absorbent layer 3. The pattern to be projected on the resin to be exposed is formed on the mask by engraving the absorbent layer 3. Thus, only the engraved zones 4 will be reflecting and will register during lithographic projection as shown by the arrows in FIG. 1.
The substrate 1 may be a silicon chip with a diameter of 200 mm. Other substrates are conceivable to take account of problems of stability under luminous flux, for example a glass with a low coefficient of expansion.
The reflector 2 is constituted by a stack of multi-layers. A high reflection factor is obtained by means of an interferential structure of multi-layers and by the right choice of materials constituting the multi-layers. Molybdenum and silicon perform very well at the 13 nm wavelength and reflection factors above 65% may be obtained with about forty multi-layers, each multi-layer being formed of a layer of molybdenum and a layer of silicon. Other layers of materials are conceivable like for example Mo/Be or again Ru/Be.
The absorbent layer 3 must be absorbent in the spectral range of the EUV, particularly around 13 nm. The materials that may be used to constitute it are numerous. Materials used in micro-electronics (like Ti, Ta, TaSi, TiW, W, Cr, TiN, Al) are preferred. For optics having a reduction factor of 4, the size of the pattern engraved in the absorbent layer will be four times greater than the pattern exposed on the circuit. Reference is made to a graticule on a scale of 4X.
To keep to the specifications associated with the exposure stage, i.e. to print on the exposed resin a pattern of good geometric definition, retaining the required dimensions over the whole exposed zone and without defects, the manufacture of the mask must meet certain conditions. The mask must have excellent flatness, typically with distortion values below 0.4 xcexcm for a diameter of 200 mm. Its coefficient of reflection must be sufficient to guarantee the dose of EUV radiation required for exposure, i.e. a coefficient of reflection above 60%. The absorbent layer must be made (deposit and engraving) in accordance with the required geometry. The surface defect density of the mask must be extremely low, of the order of some 10xe2x88x923 defects/cm2, for defects of dimensions equal to or above about 50 nm. The multi-layer and the absorbent layer constituting the mask are located in the object plane of the optical system. Consequently, any significant variation in phase or amplitude of the field reflected by the mask may potentially be replicated by the lithography process and may thus impart a crippling defect into the resin. If there are processes to repair defects in the absorbent layer, there is no satisfactory solution for repairing defects in the multi-layer reflector. This last condition remains the major problem in mastering the manufacture of masks. The best results obtained are of some 10xe2x88x922 defects/cm2.
Mask specifications, in terms of controlling shape and defects, presuppose the technological mastery of every stage in its manufacture. Masks are developed in two large stages:
manufacturing the multi-layer reflector substrate called the xe2x80x9cmask blankxe2x80x9d in the specialised literature,
mask patterning, i.e. making the absorbent layer engraved on the multi-layer reflector substrate.
Several approaches have been proposed for making the masks: a process by direct engraving of the absorbent layer, a process by lift-off (see the article by H. KINOSHITA et al., xe2x80x9cMask technology of extreme ultraviolet lithographyxe2x80x9d, SPIE vol. 3412, 358-368) and a process of the damascene type.
The process by direct engraving of the absorbent layer includes the following basic stages:
depositing the multi-layer reflector on a substrate,
possible depositing of protective and/or buffer layers (intermediate layers),
depositing the absorbent layer by a process similar to that for depositing a metal film in micro-electronics,
lithography stage (resin spreading, exposure, processing) to delimit the zones to be engraved,
selective engraving of the absorbent layer, intermediate layers being possibly necessary between the absorbent layer and the multi-layer reflector to allow selective engraving and to protect the multi-layer,
partial or total elimination by engraving of the intermediate layers, depending on the nature of said layers and on their transparency to the wavelength used, in engraved zones of the absorbent layer.
The damascene type process includes the following stages:
depositing the receive multi-layer on a substrate,
depositing and engraving another layer,
depositing the absorbent layer and eliminating said layer above the non-engraved patterns of the previous layer.
The major difficulty with current processes lies in the presence of defects on the surface of the multi-layer reflector the size of which is greater than a critical dimension. These defects originate:
in growth defects linked to defects or imperfections in the support (substrate),
in defects generated during development of the multi-layer stack (pollution by particles while depositing or growth defects at each interface of the multi-layer).
Defects in thin layers, and all the more so in multi-layers, may be generated by very small defects. The size of the defects grows, by shadow effect, with the thickness of deposit coating them as the multi-layer is developed. Initiated as defects of non-critical size, the defects may on the surface exceed the critical size. The critical size will depend on the application.
FIG. 2 is a transverse cross-section view of a substrate 5 coated with a succession of layers intended to constitute the multi-layer reflector 6 of a mask according to the prior art. The references 7 and 8 represent defects initially of non-critical size and which have developed to the point where they exceed the critical size.
To overcome this problem of the prior art, it is proposed according to the present invention to reverse the multi-layer structure so that the first layer of the multi-layer (the one made first during its development) is the one positioned on the front face (illuminated face) in the final structure of the mask and not the opposite as in prior art structures. The advantage of this solution is that the apparent surface defects in the final structure of the mask will be smaller and less numerous. Disturbance by the defects of the reflected wave will be reduced. And this because, in a reflective system of interferential multi-layers, the contribution to the total reflection of the different reflective waves by each interface of the multi-layer decreases the further from the surface the interface under consideration is located.
A first object of the invention is constituted by a structure for a lithographic reflection mask comprising a receive medium on which is fixed a reflector including at least one layer, the reflector being fixed to the receive medium in a reverse manner relative to a manufacturing medium on which it has been previously manufactured.
Preferably, the reflector is a multi-layer, for example formed by an alternation of layers of molybdenum and silicon. Other materials of the prior art may of course be used.
The reflector may be fixed to the receive medium by means of a bonding layer.
If the reflector has been manufactured on a manufacturing medium coated with an absorbent layer for the wavelength of the source of lithography radiation used, the reflector and the absorbent layer may be fixed to the receive medium in a reverse manner relative to the manufacturing medium. Possibly, the absorbent layer coating the manufacturing medium is an engraved layer according to a desired mask pattern.
Thus, the patterns of the absorbent layer of the structure for the mask according to the invention may be made according to the direct or damascene type processes previously defined. These patterns may be made either before the formation of the reflector on the manufacturing medium as shown above, or after the formation of the reflector and its transfer onto the receive medium.
The receive medium may be a substrate of vitreous silica or of another material with a low coefficient of expansion.
A second object of the invention is constituted by a manufacturing process of a structure for a lithographic reflection mask, characterised in that it comprises the following stages:
making a reflector on a manufacturing medium,
fixing a receive medium on the reflector,
at least partly eliminating the manufacturing medium.
The reflector may be made in the form of a multi-layer.
The process may include a preliminary stage of formation of an absorbent layer, for the wavelength of the source of lithographic radiation used, on the manufacturing medium before the reflector is made.
The fixing of the receive medium on the reflector may be achieved by molecular adhesion. To advantage, a bonding layer is used to fix the receive medium on the reflector.
According to an implementation variant, eliminating the manufacturing medium occurs by mechanical and/or mechanical-chemical and/or chemical thinning. Possibly, the manufacturing medium being provided, reflector side, with a barrier layer, the thinning occurs to the point of contact with the barrier layer. The manufacturing medium being eliminated using mechanical thinning, this may be stopped before complete elimination of the manufacturing medium, the elimination being then continued chemically.
According to another implementation variant, the manufacturing medium comprising, reflector side, a sacrificial layer, the elimination of the manufacturing medium is achieved by selective engraving of the sacrificial layer.
According to yet another implementation variant, the manufacturing medium is a substrate one face of which, intended to be located on the reflector side, has been implanted by gaseous radicals so as to form a layer of microcavities in the substrate and in proximity to the implanted face, the elimination of the manufacturing medium including the application of an anneal and/or mechanical force to obtain the cleavage of the manufacturing substrate along the layer of microcavities. There then remains a thin film. The barrier layer or the thin film may then be totally or partially eliminated according to the variants of the invention, i.e. totally where the transfer onto the receive medium is done with the absorbent patterns and totally or partially where the absorbent patterns are made after transfer of the reflection layer.